During the manufacture of wafers, defects can occur on their surface which later substantially impair the function of the components to be manufactured from the wafers, or make them unusable. Defects in the crystal structure of the wafers, in particular in an epitaxial layer (called “epitaxial defects”), have proven to be especially troublesome in this context.
Epitaxial defects occur, for example, in the silicon layer as a result of disruptions in crystal growth. The usual cause is excessively rapid crystal growth during manufacture of the epitaxial layer, which is deposited, for example, in a CVD process using trichlorosilane at temperatures of approximately 1150° C. Contaminants can also cause epitaxial defects.
Because of their effects on the components to be manufactured, it is of interest to prevent occurrence of the aforesaid defects or, since that is usually not possible with sufficient reliability, to know the exact number and location of the defects present on a wafer. As a rule, a maximum of approximately three defects with sizes in the range up to a few tens of μm are tolerated on a wafer having a diameter of 8 inches.
A number of methods and apparatuses for the detection of epitaxial defects are disclosed in a presentation entitled “Discrimination of defects on epitaxial silicon wafers” published in Electrochemical Society Proceedings, Vol. 97/22, pp. 438 ff.
For example, it is known to examine unpatterned wafers by means of an optical inspection device, for example a microscope. For that purpose, essentially the entire surface of a wafer to be inspected, with the exception of a very narrow edge region that is not used for the production of components, is scanned.
The image data thereby obtained are examined for the presence of defects using image data processing methods known per se. This procedure is very time-consuming, however, given the very small physical extent of the possible defects. Several hours are therefore required for analysis of the image data of a single 8-inch wafer. Thus, although unpatterned wafers can in principle be examined with sufficient accuracy for the presence of defects, this procedure is nevertheless unsuitable for checking wafers in industrial production because of the immense time requirement.
So-called scattered-light measuring instruments, with which the local scattered-light intensity on a wafer is determined, are therefore used in this context. Less time is required for this. Practical investigations, however, for example those described in the publication cited above, indicate that not all epitaxial defects can be discovered with such instruments and methods. In the investigation cited, in the best case 96% of all epitaxial defects were detected. It must also be considered in this context that with the scattered-light method it is not possible to distinguish between epitaxial defects, non-epitaxial defects, and artifacts (i.e. false defects).
This means that ultimately what is determined with a scattered-light examination is not the presence of a defect but merely an exceedance beyond a threshold value, with no possibility of concluding what that threshold value exceedance can be attributed to. Classification of the defects is thus not possible. Investigations have furthermore shown that the scattered-light method usually indicates considerably more defects than are actually present, but that on the other hand defects are also overlooked.
In the publication cited above, it was therefore suggested that a wafer first be examined with a scattered-light measuring instrument and that the locations of a threshold value exceedance be recorded so they can later be examined more closely in an optical inspection device. In the latter, a classification of the defects can simultaneously also be performed. By limiting the reinspection to the locations of a threshold value exceedance, it is in principle possible to integrate a microscopic examination, which would take too long for an examination of the entire wafer, into an inspection method that is suitable for the industrial production of wafers. The time for a reinspection at a location with a threshold value exceedance is only on the order of approximately one second. The accuracy and efficiency of the entire method thus depend on the number of threshold value exceedances identified in the scattered-light measuring instrument.